Geochemistry of Deccan Tholeiite Flows and Dykes of Elephanta Island
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geosciences Article Geochemistry of Deccan Tholeiite Flows and Dykes of Elephanta Island: Insights into the Stratigraphy and Structure of the Panvel Flexure Zone, Western Indian Rifted Margin Vanit Patel 1,2, Hetu Sheth 1,*, Ciro Cucciniello 3,* , Gopal W. Joshi 4, Wencke Wegner 5,6, Hrishikesh Samant 7, Bibhas Sen 8 and Christian Koeberl 5,6 1 Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India; [email protected] 2 Geological Survey of India, Central Region, State Unit Madhya Pradesh, Jabalpur 482003, India 3 Dipartimento di Scienze della Terra, dell’ Ambiente e delle Risorse (DiSTAR), Università di Napoli Federico II, Complesso Universitario Monte Sant’Angelo, Via Cintia 21 (edificio L), 80126 Napoli (Naples), Italy 4 Thermo Fisher Scientific India Pvt. Ltd., Delphi, B Wing, Hiranandani Business Park, Powai, Mumbai 400076, India; [email protected] 5 Department of Lithospheric Research, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria; [email protected] (W.W.); [email protected] (C.K.) 6 Natural History Museum, Burgring 7, A-1010 Vienna, Austria 7 Department of Geology, St. Xavier’s College, Mumbai 400001, India; [email protected] 8 Geological Survey of India, Northeastern Region, Shillong 793003, India; [email protected] * Correspondence: [email protected] (H.S.); [email protected] (C.C.) Received: 25 January 2020; Accepted: 24 March 2020; Published: 26 March 2020 Abstract: Elephanta Island near Mumbai is an important area for understanding the stratigraphic and structural framework of the Deccan flood basalt province in the tectonically disturbed Panvel flexure zone on the western Indian rifted margin. Elephanta exposes a west-dipping, 66–65 Ma sequence of tholeiitic lava flows and dykes. Geochemical correlations with the thick, horizontal, 66–65 Ma Western Ghats sequence to the east show that lava flows of the Khandala and Ambenali formations are present at Elephanta, with two lava flows probably being locally derived. The Elephanta tholeiites have experienced crystal fractionation and accumulation, particularly of olivine. They have "Nd(t) ranging 87 86 from +5.4 to 7.9 and ( Sr/ Sr)t from 0.70391 to 0.70784, with most tholeiites little contaminated − by continental lithosphere, probably lower crust. Field and geochemical data indicate a normal fault along the central part of Elephanta with a 220 m downthrow, consistent with a domino-type block-faulted structure of Elephanta, and the surrounding area as previously known. Seventeen of the 20 analyzed Elephanta intrusions, striking ~N–S, belong to the Coastal dyke swarm of the western Deccan province. Several of these are probable feeders to the Ambenali Formation in the Western Ghats sequence, requiring reconsideration of the current view that the voluminous Wai Subgroup lavas of the Western Ghats were erupted without organized crustal extension. East–west-directed extensional strain was already active at 66–65 Ma along this future (62.5 Ma) rifted continental margin. A young (~62 Ma) ankaramite dyke on Elephanta Island is a probable feeder to the Powai ankaramite flow in the 62.5 Ma Mumbai sequence 20 km to the northwest. Keywords: volcanism; flood basalt; volcanic rifted margin; tholeiite; geochemical stratigraphy; Deccan Traps; India Geosciences 2020, 10, 118; doi:10.3390/geosciences10040118 www.mdpi.com/journal/geosciences Geosciences 2020, 10, 118 2 of 35 1. Introduction: Flood Basalts, Rifted Continental Margins, and Monoclinal Flexures Volcanic rifted margins [1,2] form during continental break-up and the birth of new ocean basins, and are associated with voluminous flood basalt magmatism (both extrusive and intrusive) and pronounced extensional tectonics. Major flood basalt provinces located on rifted continental margins are the Karoo province of southern Africa, the Paraná province of South America, the East Greenland and West Greenland provinces, and the Deccan province of India [3,4]. The coastal edges of these continental flood basalt (CFB) provinces show monoclinal flexure zones, in which the kilometers-thick, flat-lying flood basalt sequence of the province’s interior shows significant tectonic dips toward the newly formed ocean [5–7]. Rifted continental margins with flood basalts thus provide excellent opportunities to study the interplay of magmatism and extensional tectonics on one hand [8–10], and the dynamics of uplift and erosion of the rift shoulders on the other [1]. The Deccan CFB province, presently covering ~500,000 km2 in western and central India, was rapidly constructed at 66–65 Ma (e.g., [11,12]) and is best developed in the Western Ghats (WG hereafter) escarpment (Figure1a). Here, extensive geochemical stratigraphic work over a north–south distance of ~500 km has divided the volcanic sequence into three subgroups and eleven formations with a total stratigraphic thickness of ~3.4 km (e.g., [13–15], Table1). Three major dyke swarms also outcrop in the Deccan province [16–19] (Figure1a), namely the ~ENE–WSW-trending Narmada-Tapi swarm in the north-central part of the province, the ~N–S-trending Coastal swarm on the Konkan Plain (the narrow coastal strip between the WG escarpment and the Arabian Sea), and the Nasik-Pune swarm with dykes of several trends in the WG region. The Narmada-Tapi and Coastal dyke swarms are the feeders of some lower and middle stratigraphic formations of the WG sequence, whereas the Nasik-Pune dyke swarm mainly fed lavas of the middle and upper formations [17–19]. Several lava flow sequences located in the central, northern, and northeastern parts of the Deccan province broadly correlate with the WG sequence [20–22], implying that large-volume lava flows of the WG region may have travelled hundreds of kilometers to these distant areas. The Deccan CFB eruptions were followed by the India–Laxmi Ridge–Seychelles breakup at 62.5 Ma [23,24], with significant Deccan magmatism occurring syn-breakup and continuing post-breakup up to ~61 Ma in Mumbai and the Seychelles [23,25]. Table 1. Geochemical stratigraphy of the Western Ghats region, Deccan Traps 87 86 143 144 206 204 Group Subgroup Formation Polarity ( Sr/ Sr)t "Nd (t) ( Nd/ Nd)t Pb/ Pb Desur * (~100 m) N 0.7072–0.7080 –5.9 to 10.4 0.512251–0.512020 na − Panhala (>175 m) N 0.7046–0.7055 +3.8 to 0.5 0.512748–0.512527 na − Wai Mahabaleshwar (280 m) N 0.7040–0.7055 +5.6 to 7.4 0.512840–0.512174 16.65–18.18 − Ambenali (500 m) R 0.7038–0.7044 +8.3 to +2.5 0.512978–0.512681 17.53–18.48 Poladpur (375 m) R 0.7053–0.7110 +3.3 to 11.0 0.512722–0.511989 17.40–19.32 Deccan − Basalt Bushe (325 m) R 0.7078–0.7200 –7.5 to 19.4 0.512169–0.511559 18.33–22.85 Lonavala − Khandala (140 m) R 0.7071–0.7124 –1.0 to 20.4 0.512502–0.511507 16.69–19.26 − Bhimashankar (140 m) R 0.7067–0.7077 +0.2 to 5.9 0.512563–0.512251 19.60–20.92 − Thakurvadi ** (650 m) R 0.7067–0.7224 –2.9 to 14.0 0.512404–0.511835 17.28–20.28 Kalsubai − Neral (100 m) R 0.7062–0.7104 –2.5 to 15.0 0.512425–0.511784 16.68–19.95 − Jawhar-Igatpuri (>700 m) R 0.7085–0.7128 –3.2 to 8.5 0.512389–0.512117 19.12–22.52 − Notes: * The Desur is considered by some workers as a “Unit” of the Panhala Formation. ** The Sr-isotopic range for most of the Thakurvadi Formation lavas is 0.7067–0.7112, but a single flow in the formation (Paten Basalt) has 87 86 anomalous, broadly Bushe-like values of ( Sr/ Sr)t = 0.7224 and "Nd(t) = 15.4. Formation names and thicknesses and isotopic ranges are based on [26] and references therein, [27–29], and include− the new lava flow analyses in [19]. N = normal magnetic polarity, R = reverse magnetic polarity. Note that the R-N polarity transition (29R to 29N) occurs not at the Ambenali-Mahabaleshwar contact but a little above it [30]. All Sr and Nd isotopic ratios are initial ratios age-corrected to 65 million years. Pb isotopic ratios are present-day values. na = not analyzed. 40Ar/39Ar and U-Pb zircon ages available for several of these stratigraphic units are given in [31]. Geosciences 2020, 10, 118 3 of 35 Figure 1. Maps of the Deccan Traps (a), a large part of the Konkan Plain in the Western Deccan Traps (b), and the Mumbai City and Elephanta areas of the Konkan Plain (c), each with important features and localities mentioned in the text marked. In (a), the dashed blue line is the Western Ghats escarpment (WGE), NTDS is the Narmada-Tapi dyke swarm, CDS the Coastal dyke swarm, and NPDS the Nasik-Pune dyke swarm ([8] and references therein). (b) shows the map and west–east cross-section (drawn just south of Alibag) of the stratigraphic subgroups of the Western Ghats sequence (based on [18]). The westward extent and continuity of the WG sequence are less well known because the Konkan Plain (Figure1b) is a structurally disturbed region, being part of the 62.5 Ma rifted continental margin [23]. A major tectonic structure known as the Panvel flexure (Figure1b) is found here, in which the Deccan volcanic sequence shows a significant seaward dip with block faulting [5,32–34]. The island of Mumbai in the westernmost Deccan province (Figure1a,c) shows a compositionally diverse volcanic sequence (tholeiite, spilite, and rhyolite lava flows and pyroclastics) with mafic and felsic intrusions, dipping west at ~18◦ (e.g., [35–38]). The Mumbai tholeiitic flows and dykes show geochemical differences with the WG sequence [39], and the entire Mumbai sequence is 62.5 Ma in age (Danian), as known from palaeontological evidence on inter-lava sedimentary and pyroclastic beds [40,41] and 40Ar/39Ar dating [23,42].